10.  

Of the 800 students at a certain college, 250 students live on campus and are more than 20 years old.  How many of the

800 students live on campus and are 20 years old or less?

(1)  640 students at the college are more than 20 years old.

(2)  60 students at the college are 20 years old or less and live off campus.

请问怎么做啊 ？哪位大侠教教，我是数学白痴